New Mexico Geological Society Annual Spring Meeting — Abstracts

Low-angle normal faults (detachments; LANFs; dip <30°) within evaporate-rich Permian strata are widespread along the eastern flank of the Rio Grande rift near Socorro, New Mexico. First recognized by Smith (1983), this fault system has gone largely unstudied due to the complex structure and stratigraphy in the area. It is the only known LANF system in the Rio Grande rift that initiated at low angles; others appear to have been rotated to gentle dips. The system is composed of younger-on-older faults with stratigraphically-controlled flats (commonly in gypsum-rich intervals) and upper- and lower-plate ramps, most of which cut down section to the east, consistent with top to the east slip. The faults are generally confined to the upper Yeso formation but some also occur in the stratigraphically higher Glorieta and San Andres formations. The detachments excise stratigraphy and, in many places, juxtapose broadly-warped hanging-wall rocks over foot-wall rocks that are folded on much shorter wavelengths. New detailed mapping shows that the detachment system continues along the N and E flanks of the Sierra Larga in the most extensive known upper-plate flat. The internally extended upper-plate, as shown by roll-over anticlines, is limited to the western Sierra Larga and areas farther W. Reconnaissance suggests the system likely extends 30-40 km E, where relatively undeformed Permian strata overlie older tightly-folded and over-thickened Yeso rocks in the Chupadera anticline.

This fault system may have originated as a Laramide thrust and/or gravity slide decollement, as suggested by tightly folded and locally thrust-faulted and overturned strata in the gypsiferous units of the Yeso Formation.  These folds are cut by intermediate dikes, one of which, in western Sierra Larga, yields an Oligocene (⁴⁰Ar/³⁹Ar total gas age of 34.68 +/- 0.11 Ma). This dike is, in turn, cut by detachment-related faults. Traced S, the detachment merges with the E-down, listric Bustos normal fault, which cuts rocks as young as late Oligocene, suggesting that the detachment system was reactivated by normal-sense slip, which explains the many reverse-drag roll-over anticlines in the upper plate.  The detachment system is commonly cut by younger, steeper normal faults.

Alternatively, the fault system may have formed from spreading related to synvolcanic loading beneath the volcanic edifices that developed along the Sierra uplift in the middle Eocene-late Oligocene. The presently known N-S extent of the detachments is similar to that of the broad Prairie Spring and Chupadera anticlines and a subsurface halite "tongue" in the Yeso Formation to the east. Salt casts and dissolution structures within Yeso outcrops suggest the halite "tongue" may have extended farther W, providing a weaker medium for slip than gypsum or anhydrite. The over-thickened Yeso within broad folds to the E may represent the blind "toe" of a gravity driven slide.

References:

1. Smith, C.T., 1983, Structural problems along the east side of the Socorro constriction, Rio Grande rift: New Mexico Geological society, 34th Fall Field Conference Guidebook, P. 103-109